When 'Bramhan' or 'Chaitanya' (The Spirit) is covered with 'Maya' (illusion) is called 'Ishvara' and 'Ishvara' freed from 'Maya' is Bramhan or Chaitanya. Three types of 'Gunas' (quality) Ishvara/God can possess are Sattva, Rajas and Tamas. Vishnu originated from Sattva, Bramha from Rajas, Siva from Tamas and all other Gods and Goddesses also originated from Ishvara. Ishvara or Bhagavan is same the difference is only in their karma (work). Ishvara - 'I' pronounced as Ee in Sanskrit denotes power and 'shvar' means breathing; that is, practicing breathing by pulling in and out with power in an 'Antarmukhi' (introvert) way (Pranayam) when done for years, a state of calmness / absolute stillness prevails and that is known as Ishvara. Bhagavan - 'Bhaga' means Yoni / Kutastha and Van (also known as arrow in Sanskrit) means Breath; that is, when breathing like arrow is shoot in an 'Antarmukhi' (introvert) way for years, a state of absolute calmness which gets generated after staying in Kutastha refers as Yoni, is known as Bhagavan. Restlessness of mind is 'Jiva' (living being) and absolute stillness of mind is Bhagavan. Bhagavan is always awake, full of consciousness, a immense powerfully substance occupying the whole Universe. Any type of existence, workforce, movement, knowledge or mastery is not at all possible without Him. When He is 'Nirguna' (inert / does not possess any quality/Guna) and inactive, He is known as Bramhan and when He is visible possessing any quality / Guna is known as Bhagavan. His wish and power of 'Vasana' (to achieve or get or fulfill something) is 'Prakriti' (Nature) or 'Mahamaya'. One who is Bhagavan, is Atma and also Bramhan. Depending upon the mental state of devotee, when Bramhan or Atma gets into a conscious bodied state becomes Bhagavan.
Bhagavan is undivided, Supreme Cosmic Consciousness. Sometimes in India we say, "Wherever there is Jiva, there is Siva (referred as God / Bramhan)". Jiva is multiple and many but Siva is only one. Restless state of mind is Jiva and absolute calmness of mind is Bhagavan. Hence if Siva does not exist we can't ever think of any Jiva, all the integral soul of Jiva is Siva. Actually Jiva and Siva is same. If we put thousands of pot full of water we can see thousands of reflection of sun, actually sun is only one and not many.